Operation of mercury-cathode cells

ABSTRACT

A process for the reduction of thick mercury on the baseplate of a mercury cell which comprises dispersing water or an aqueous medium (preferably brine) continuously or intermittently at one or more stations along the line of amalgam flow without interruption of the electrolysis. It is preferred to inject water or brine in part or whole of the stream of amalgam returning from the denuder to the cell, for example by means of an emulsifier located between the denuder and the cell or by injecting water into the amalgam pump.

United States Patent 1191 Appl. No.: 465,416

Foreign Application Priority Data May 17, 1973 United Kingdom 23356/73References Cited UNlTED STATES PATENTS 7/1900 Entz 204/99 4/1934 Ramseyet a1. 204/250 Lee et al. Sept. 16, 1975 [54] OPERATION OFlVlERCURY-CATHODE 2,307,835 1/1943 Gardiner 204/99 CELLS 3,560,3552/1971 Shibata et a1. 204/99 3,627,652 12/1971 De Nora 204/99 1Inventors: Denis Leslie Norbum, both of 3,645,866 2/1972 Volkov 204 99Runcorn, England [73] Assignee: Imperial Chemical Industries PrimaryExaminer-R Andrews Limited, London, England Attorney, Agent, orFirmCushman, Darby &

Cushman [22] Filed: Apr. 29, 1974 [5 7] ABSTRACT A process for thereduction of thick mercury on the baseplate of a mercury cell whichcomprises dispersing water or an aqueous medium (preferably brine)continuously or intermittently at one or more stations along the line ofamalgam flow without interruption of the electrolysis.

It is preferred to inject water or brine in part or whole of the streamof amalgam returning from the denuder to the cell, for example by meansof an emulsifier located between the denuder and the cell or byinjecting water into the amalgam pump.

11 Claims, 8 Drawing Figures PATENTED SEP I 6 I975 SHEET 2 BF 4 PATENTEHSEP 3 8 E92 5 snmaurg The present invention relates to the operation ofmercury-cathode cells for the electrolysis of alkalimetal chloridesolutions. More particularly it relates to stallations of cells for themanufacture of chlorine and caustic alkali by the electrolysis ofalkali-metal chloride solution, wherein the said solution iselectrolysed while flowing between the lower faces of an array ofgraphite or metal anode plates and a flowing liquid cathode, which ismaintained by feeding in mercury or dilute alkali-metal amalgam at oneend or one side of the cell and withdrawing amalgam enriched inalkali-metal at the opposite end or side of the cell. Chlorine liberatedat the anodes is continuously removed from the top of the cell and theliberated alkali-metal, which collects in the flowing amalgam cathode iscontinuously removed in the enriched amalgam and converted to caustical' kali by reaction of the enriched amalgam with water in a soda cell,usually called a denuder, from which dilute amalgam is recirculated bymeans of a pump to the electrolytic cell.

A well-known problem in operating such cells is the build-up of depositsof thick mercury, sometimes referred to as mercury butter on thebaseplate of the cell. The mechanism of thick mercury formation is notfully understood. It is thought to be influenced by the presence oftrace impurities in the brine electrolyte, but even with the bestpracticable attention to purification of the feed-brine thick mercurydeposits can build up with prolonged operation of the cell and theproblem has tended to become now acute with the high-current-densityoperation which has been practised in recent times.

Thick mercury deposits can cause current shorts between the anodes andthe cathode amalgam. Besides reducing the current efficiency of theprocess such shorts can cause serious damage, especially to the coatedmetal anodes which have recently begun to replace conventional graphiteanodes.

We have now found that the build-up of thick mercury on the cellbaseplate can be reduced by dispersing water or an aqueous medium in theamalgam cathode without interruption of the electrolysis.

According to the present invention we provide a process for thereduction of thick mercury on the cell baseplate of a mercury cell whichcomprises dispersing water or an aqueous medium continuously orintermittently at one or more stations along the line of the amalgamflow without interruption of the electrolysis.

The preferred aqueous medium is brine, for example the aqueouselectrolyte of the cell.

The accompanying drawings illustrate schematically seven suitableembodiments of apparatus for putting the process of the invention intopractice.

Each of the FIGS. 1 to 6 shows an isometric projection as seen from oneside edge of the baseplate the interior of a portion of amercury-cathode cell in which the cathode-amalgam flow is along thelength of the cell. FIG. 7 shows a schematic vertical section along thelength of a mercury-cathode cell with its midportion cut out and theexternal amalgam circulation. FIG. 8 shows a detail of- FIG. 7.

In each of the FIGS. 1 to 6 the cell baseplate l carries the flowingamalgam cathode 2 and above this is the flowing aqueous electrolyte 3.The anodes which lie above the amalgam layer in the electrolyte 3 havebeen omitted from the drawings for the sake of clarity.

In FIG. 1 an inclined baffle 4 is fixed across the line of amalgam flowwith its lower edge spaced from the baseplate l and below the normallevel of the amalgam, and with its upper portion lying in the aqueouselectrolyte 3. The baffle 4 may suitably consist of a steel core coatedwith a chlorineresistant plastics material. Because of the interferenceto flow of the two liquids by the inclined baffle and the very highspecific gravity of the amalgam layer, a portion of the aqueouselectrolyte is drawn under the baffle with the amalgam, turbulence iscreated and sufficient electrolyte is caused to disperse in the amalgamto cause decomposition of any thick mercury tending to deposit on thebaseplate for an appreciable distance downstream from the baffle.Depending on the length of the cell and other parameters, such as theslope of the baseplate and current density, a plurality of baffles 4spaced apart along the cell may be required to keep the whole length ofthe cell free from thick mercury deposits.

In FIG. 2 a roller 5 replaces the baffle 4 of FIG. 1. The roller isrotatable about an axle 6 carried in bearings (not shown), which may befixed to the baseplate or the walls of the cell. The roller is spacedaway from the baseplate and is immersed partly in the cathode amalgam 2and partly in the aqueous electrolyte 3. R0- tation of the rollerscaused by'the flowing amalgam layer draws a portion of the aqueouselectrolyte beneath the roller and produces the required dispersion ofelectrolyte in the amalgam. Additional rollers 5 may be fixed atintervals down the line of the amalgam flow.

In FIG. 3 an electromagnetic transducer 7, working preferably at anultrasonic frequency, is placed across the line of amalgam flow toreplace the baffle 4 of FIG. 1 and the roller 5 of FIG. 2. The positionof the'lower surface of the transducer is adjusted so as to oscillateabout a mean position defined by the interface between the amalgam layerand the aqueous electrolyte when the transducer is not in operation. Thetransducer may be operated continuously to maintain a dispersion ofaqueous electrolyte in the amalgam downstream from the position of thetransducer or it may be operated for 7 short periods, eg 10-minuteperiods, intermittently so as to prevent the build-up of thick mercurydeposits to any significant extent. Additional transducers 7 may beinstalled at intervals down the line of the amalgam flow as necessary tomaintain any length of baseplate free from thick mercury deposits.Furthermore, each transducer shown in FIG. 3 as a single transducer 7may be replaced by a plurality of smaller independently actuatedtransducers operating side by side across the line of amalgam flow. Thesuspension arrangements and power supply lines for the transducers,indicated as 7a, may suitably pass through the cover of the cell (notshown).

In FIG. 4 is shown an alternative arrangement employing electromagnetictransducers for dispersing aqueous electrolyte in the flowing amalgamcathode. Here a transducer 26 is installed above the cover of the cell(not shown) and the mechanical vibrations generated by the transducerare conveyed into the cell by means of a bar 27 passing through the cellcover and reaching to the interface between the amalgam cathode 2 andthe aqueous electrolyte 3. Although for clarity only one transducer isshown in FIG. 4, generally a plu rality of transducers 26, each with itsattached bar 27, v

will be installed in a spaced row across the cell so as to disperseaqueous electrolyte in the amalgam cathode over the full width of thecell. If desired, the cell cover may be provided with a row of openableports through which the bars 27 may be introduced only when it isdesired to treat the amalgam cathode for a short period. After thetreatment the transducers may be moved to another station in the samecell, where another row of openable ports is provided, or to a differentcell. The bars 27 must be made of chlorine-resistant material or must becoated with a chlorine-resistant material. Most suitably the bars areprovided with an electricallyinsulating coating so as to avoid thepossibility of shorting between the anodes and the cathode of theworking cell when the bars are being introduced and removed.

FIG. 5 shows an arrangement for introducing water or brine into theamalgam cathode by injection from a series ofjet orifices placed abovethe amalgam cathode 2 in the cell. An inlet pipe 28 carries a series ofbranches 29, each of which is terminated by a jet orifice 30 (one onlyshown). The pipe 28 may be fixed within the cell above the aqueouselectrolyte 3; alternatively it may lie outside the cell and thebranches 29 will then pass into the cell through the cell cover (notshown). The jet orifices 30 may be placed above the aqueous electrolyteas shown in the drawing or alternatively they may be placed within theaqueous electrolyte. The end 31 of pipe 28 is closed and water or brineis fed in at the end 32 under adequate pressure so that it issues fromthe jet orifices 30 with sufficient velocity to penetrate through theaqueous electrolyte 3 and disturb the amalgam cathode 2. It may suitablybe arranged for the high-speed jets of water or brine thus produced topass through holes or slots provided in the anode plates or through gapsprovided between neighbouring anodes.

FIG. 6 shows an arrangement for injecting water or brine directly intothe amalgam cathode. An inlet pipe 33 is fixed in the cell, preferablyabove the level of the flowing electrolyte 3 as shown, and carries aseries of branches 34, each of which terminates in a jet orifice 35(only one shown) within the flowing amalgam cathode layer 2. The end 36of pipe 33 is closed and water or brine is fed in at the end 37 undersufficient pressure to emerge into the amalgam cathode layer through thejet orifices 35.

In general sufficient water remains dispersed in the mercury returningto the chlorine cell from the denuder to prevent the build-up of thickmercury deposits for a short distance downstream from the mercury inletof the cell. It is not therefore necessary to disperse additional wateror brine in the amalgam cathode near the mercury inlet and the firststation downstream from the inlet at which any of the devices shown inFIGS. 1 to 6 is installed will generally be chosen to lie just beforethe position at which the build-up of thick mercury deposits is known tostart during conventional operation of the cell.

In FIG. 7 is shown another arrangement of apparatus for dispersing wateror brine in the flowing cathode amalgam within the cell in accordancewith the invention. The cell baseplate 1 carries the flowing amalgamcathode 2 and above this is the flowing brine electrolyte 3. 8 are theend walls ofthe cell through which pass the brine inlet 9, thespent-brine outlet 10, an inlet 11 for mercury or dilute amalgam and anoutlet 12 for enriched amalgam. The cell cover 13 is provided with anoutlet 14 for chlorine gas, and the electrical connections 15 passthrough the cover to the anodes 16. Enriched amalgam is continuouslyremoved from the cell at 12 to the denuder 17, where it is decomposed bya supply of water (not shown), and a stream of dilute amalgam flows fromdenuder 17 through water-wash box 18 and pump 19 back to the inlet endof the cell through pipe 20. The main stream of dilute amalgam returnsto the cell through valve 21 and inlet 11. As discussed hereinbefore,the mercury stream returning to the cell from the denuder by way of pipe20 contains some water dispersed in it. According to the embodiment ofthe invention illustrated in FIG. 7, a small fraction of this wetmercury stream, controlled by valve 22 in conjunction with valve 21, isfed by way of pipe 23 to a spreader-bar 24 which lies across the line ofamalgam flow and dips into the amalgam cathode just upstream from theposition where build-up of thick mercury deposits is known to startduring conventional operation of the cell. The detail of thespreader-bar is shown in side elevation in FIG. 8. The stream of wetmercury delivered through pipe 23 passes into the hollow spreader-bar 24and enters the flowing amalgam cathode through a series of orifices 25.Additional spreader-bars 24 connected to branches (not shown) from pipe23 may be installed in the cell at intervals further down the line ofamalgam flow to maintain any length of baseplate free from thick mercurydeposits.

As a modification of the embodiment of the invention discussed in thepreceding paragraph, additional water or brine (preferably unchlorinatedfeed-brine) may be dispersed in the mercury returning to the cell fromthe denuder, for instance by installing an emulsifier fed with thechosen aqueous medium either in the total mercury stream flowing in line20 of FIG. 4 as indicated at A or in the minor stream flowing in line 23as indicated at B. This modification has the advantage of compensatingfor the progressive separation of dispersed water from a mercury streamas it moves along the pipelines, caused by the great difference inspecific gravities, and ensures a higher concentration of aqueous phaseat the points of application. If desired, when a plurality ofspreader-bars 24 is installed in the cell, a separate emulsifier B maybe installed in each of the branches of line 23 which feed theindividual spreaderbars.

By the term emulsifier, we mean a mechanical device consisting ofrotating vanes fitting within a stationary perforated screen, and whichis positioned across the interface between the amalgam and the water tobe added so as to draw water into the amalgam when the vanes arerotated.

The invention is further illustrated but not limited by the followingExamples.

EXAMPLE I The rate of formation of thick mercury content in a mercurycell was determined as follows. A vertically adjustable copper probe(diameter 3 mm) was inserted through a hole in the cell cover andscrewed down towards the baseplate. The probe was connected by means ofan electric circuit including a voltmeter to the cell baseplate. A firstreading on a micrometer gauge was taken when the probe made contact withthe upper 8 mm) indicated the thickness of the mercury film and could bemeasured to an accuracy of 1.005 mm. Sevnute, over five periods of 5 to'9 days each, but without the emulsifier.

The observed mean rates of formation of thick men cury at differentpositions along the cell, with and with 5 out the emulsifier. are shownbelow:

eral measurements eg 20 to 30, were taken at different positions alongthe cell and the mean thickness of the mercury film in each position wascalculated. The measurements were repeated at regular intervals'over 5periods of 5 to 6 days each and the rate of increase of the thickness ofthe mercury film was equated with the rate of formation of thickmercury.

A mercury cell operating with a current of 180,000 amp was fed with 4 mlhour of 25.5 percent 7w sodium chloride brine. The mean mercury flowrate was 53 litres/minute. Water was injected into the pump 19 (shown inFIG. 7) at a rate in the range 60 to 95 litres/- hour.

For the purposes of comparison, the mercury cell was operated under thesame conditions of brine flow and current, with a mean mercury flow rateof 47 litres/minute, but without water injection. The measurements werecarried out seven periods of 5 to 8 days each.

The observed mean rates of formation of thick mercury at differentpositions along the cell, with and without water injection, are shownbelow:

Distance along Mean rate of formation of The water content of theamalgam and the particle size of the water was measured by allowingwater particles to rise to the surface of the amalgam and measuring theamount of water collecting at the surface at various time intervals.When there was no water injection, the mercury feed to the cellcontained 0.02 to 0.1 ppm by weight of water whose particle size wasless than 12 micron Stokes diameter; when water was injected, themercury feed contained 0.2 to 0.6 ppm by weight of water whose particlesize was less than 12 micron Stokes diameter. These measurementsconfirmed that injection of water into the pump had produced anincreased amount of dispersion of water in the amalgam feed to the cell(there was already some water present in this amalgam).

EXAMPLE 2 An emulsifier (as shown at A in FIG. 7) was incorporated inthe mercury feed to the cell. The mercury cell was operated under thesame conditions of brine flow and current as in Example I. with a meanmercury flow rate of 62 litres/minute, over five periods of 5 to 9 dayseach, but using the emulsifier instead of water injection into the pump.The emulsifier, which was of the type described hereinbefore wasmanufactured by Silverson Machines Limited, England.

For purposes of comparison, the mercury cell was operated under the sameconditions of brine flow and current, with a mean mercury flow rate of10 litres/mi- Distance along Mean rate of formation of the cell (feet)thick mercury (mm/day) with emulsifier without emulsifier 3l O.l45 0.284

Measurements as described in Example 1 showed that the emulsifierproduced an increased amount of dispersion of water in the mercury feedto the cell. Thus, when the emulsifier was not in use, the mercury feedto the cell contained 0.1 to 0.4 ppm by weight of water whose particlesize was less than 12 micron Stokes diameter; when the emulsifier wasused, the mercury feed contained 1 to 4 ppm by weight of water whoseparticle size was less than 12 micron Stokes diameter.

What we claim is:

1. A process for the reduction of thick mercury on the cell baseplate ofa mercury cell which comprises dispersing water or an aqueous mediumwithin the body of the amalgam intermittently or continuously at one ormore stations along the line of the amalgam flow without interruption ofthe electrolysis.

2. A process as claimed in claim 1 wherein the aqueous medium is brine.

3. A process as claimed in claim 1 wherein water or the aqueous solutionis dispersed in part or whole of the stream of amalgam returning from adenuder to the cell.

4. A process as claimed in claim 3 wherein water or the aqueous solutionis dispersed in part or whole of the stream of amalgam by means of anemulsifier located between the denuder and the cell.

5. A process as claimed in claim 3 wherein the water or the aqueoussolution is dispersed in the whole of the stream of amalgam by injectingwater into a pump for the amalgam located between the denuder and thecell.

6. A process'as claimed in claim 3 wherein at least a portion of thestream of amalgam containing dispersed water is introduced along theline of amalgam flow within the cell.

7. A process as claimed in claim 2 wherein the dispersion of brine inthe amalgam is effected in the cell by means of an inclined baffle fixedacross the line of amalgam flow with its lower edge spaced from thebaseplate and positioned below the normal level of the amalgam cathodeand with its upper portion lying in the aqueous electrolyte.

8. A process as claimed in claim 2 wherein the dispersion of brine inthe amalgam is effected in the cell by means ofa roller which isrotatable about an axis transverse to the line of amalgam cathode flowand which is spaced away from the baseplate so as to be immersed partlyin the amalgam cathode and partly in the aqueous electrolyte.

9. A process as claimed in claim 2 wherein the disper sion of brine inthe amalgam cathode is effected in the cell by means of a source ofmechanical vibration.

10. A process as claimed in claim 1 wherein water or 11. A mercury cellhaving means for reducing the brine is dispersed in the amalgam byinjecting a stream build-up of thick mercury on the cell baseplate whichof brine or water into or above the aqueous electrolyte operates usingthe process of claim 1. or into the amalgam cathode itself.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3.905.880 Dat dSept. 16. 1975 Patent No.

Inventor(s) Denis Lee, and Leslie Norburn It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

IN THE HEA DING At paragraph [30] concerning Foreign ApplicationPriority Data please change the UK Application Number 3356/73" to read--23536/73--- Signed and Scaled this seventeenth Day Of February 1976[SEAL] Attest:

C. MARSHALL DANN Commissioner ufPaIents and Trademarks RUTH C. MASONArresting Officer

1. A PROCESS FOR THE REDUCTION OF THICK MERCURY ON THE CELL BASEPLATE OFA MERCURY CELL WHICH COMPRISES DISPERSING WATER OR AN AQUEOUS MEDIUMWITHIN THE BODY OF THE AMALGAM INTERMITTENTLY OR CONTINUOUSLY AT ONE ORMORE STATIONS ALONG THE LINE OF THE AMALGAM FLOW WITHOUT INTERRUPTION OFTHE ELECTROLYSIS.
 2. A process as claimed in claim 1 wherein the aqueousmedium is brine.
 3. A process as claimed in claim 1 wherein water or theaqueous solution is dispersed in part or whole of the stream of amalgamreturning from a denuder to the cell.
 4. A process as claimed in claim 3wherein water or the aqueous solution is dispersed in part or whole oFthe stream of amalgam by means of an emulsifier located between thedenuder and the cell.
 5. A process as claimed in claim 3 wherein thewater or the aqueous solution is dispersed in the whole of the stream ofamalgam by injecting water into a pump for the amalgam located betweenthe denuder and the cell.
 6. A process as claimed in claim 3 wherein atleast a portion of the stream of amalgam containing dispersed water isintroduced along the line of amalgam flow within the cell.
 7. A processas claimed in claim 2 wherein the dispersion of brine in the amalgam iseffected in the cell by means of an inclined baffle fixed across theline of amalgam flow with its lower edge spaced from the baseplate andpositioned below the normal level of the amalgam cathode and with itsupper portion lying in the aqueous electrolyte.
 8. A process as claimedin claim 2 wherein the dispersion of brine in the amalgam is effected inthe cell by means of a roller which is rotatable about an axistransverse to the line of amalgam cathode flow and which is spaced awayfrom the baseplate so as to be immersed partly in the amalgam cathodeand partly in the aqueous electrolyte.
 9. A process as claimed in claim2 wherein the dispersion of brine in the amalgam cathode is effected inthe cell by means of a source of mechanical vibration.
 10. A process asclaimed in claim 1 wherein water or brine is dispersed in the amalgam byinjecting a stream of brine or water into or above the aqueouselectrolyte or into the amalgam cathode itself.
 11. A mercury cellhaving means for reducing the build-up of thick mercury on the cellbaseplate which operates using the process of claim 1.